Wheeled vehicles depend for support, acceleration, deceleration and control on the contact forces acting between the periphery of the traction wheels and the road surface. More precisely, the contact forces are present between the periphery of the tires or other devices surrounding the circumference of the traction wheels and the substances covering the road surface. There is a basic and unresolved problem inherent in providing the appropriate magnitude and direction of the contact forces under all conditions that may arise in the course of utilizing the vehicle.
Traction wheels are defined as those which transmit the mechanical power existing in rotary motion form into forward-acting linear propulsion power for accelerating the vehicle or to balance the passive friction forces which otherwise would tend to decelerate the vehicle from its steady motion. Traction wheels (as well as other wheels not used for transmitting forward-acting propulsive power) also serve for transmitting braking power at the discretion of the driver, when deceleration of the vehicle is necessary. Except for the fact that braking power can be transmitted by traction wheels as well as by others not used for forward traction, the braking forces are in all respect similar to the forward traction forces but act in the direction opposed to the latter.
When the vehicle is not moving in a straight line or when the longitudinal axis of the vehicle is inadvertently or accidentally out of alignment with its intended trajectory, forces other than those required for propulsion or braking arise. These act in directions perpendicular to the traction and/or braking forces and are aimed at countering the centrifugal forces arising from the curvilinear motion of the vehicle, or at reestablishing the alignment between the longitudinal axis of the vehicle and its intended trajectory. For the purpose of the present invention, the contact forces described herein are called "control forces", to distinguish them from the "propulsive" or "braking" forces described above.
While the weakening or disappearance of propulsive forces is a serious matter, insofar as the safe and efficient operation of the vehicle is concerned, the absence or loss of braking forces, or the absence or loss of control fores can result in catastrophic consequence for the vehicle, its driver and its occupants. For these reasons, vehicle designers provide all practical devices that ensure that all three categories of contact forces (propulsive, braking and control) are maintained under all expected driving, weather and road conditions.
In the most general mode of operation, the wheels are equipped with inflatable elastic tires. The peripheral surface of the tires is reinforced with additional layers of rubber-like material, designed to resist wear due to friction between the road surface and the tire periphery. These layers are also impressed with grooves and indentations called "treads" of many different designs, but all aimed at increasing the tire's friction force limit. When this limit is exceeded by the force caused by the driving or braking torque imposed on the wheel or by the control forces defined above, the tire will "slide" or "skid", that is relative motion takes places between the point of contact of the wheel with the road surface and the corresponding point of the road surface itself. As earlier stated, under these conditions, the acceleration, the deceleration and the directional control capabilities of the vehicle are impaired.
Several design concepts have been proposed to avoid the loss of friction between the wheel periphery and the road surface. The simplest and the most popular is to improve the tire tread design, so as to oppose both peripheral sliding (along the tangent to the cylinder defining the periphery of the tire and in the direction parallel to the rotary motion of the wheel) or transversal sliding (along the perpendicular to this rotary motion). The effectiveness of the tire tread in preventing peripheral or transversal skidding much depends on the tread design (depth, orientation and spacing of the grooves) but mostly on the characteristics of the road surface. Thus, loose sand, mud, soil, dust, oil, water, snow, slush or ice modify the nature and the magnitude of contact forces, and special tire designs are necessary to provide the capability of maintaining adequate control of the vehicle. Such designs could include specially deep treads characteristic of "snow tires" or metal studs incorporated in the tread material ("studded tires"). While offering a measure of improvement, these design concepts are objectionable on account of tire noise on smooth roads and of wear imposed on the road surfaces, respectively. They also prove essentially useless in the presence of deep snow, semi-frozen slush, deep mud or hard-frozen icy road conditions.
Snow-chains are technically acceptable for driving under extreme road conditions. They are loosely tied around the periphery of the tires so as to interpose one or several chain links between the tire periphery and the snow-covered road surface. The weight of the vehicle impresses the chain link(s) in to the relatively soft snow surface so that the latter increase the contact forces (peripheral or lateral) required to cause relative motion between the chain links and the snow-covered road surface layer. Through other elements of the chain, the increased contact forces are transmitted to the wheel itself, providing thereby propulsive or braking or lateral control forces to the vehicle. For long stretches of uniformly snow-covered roads, snow-chains provide an effective means for enhancing or recovering the contact forces between the vehicle and the road which otherwise would be lost through the slippery nature of the road surface. Nonetheless, snow-chains are less than fully satisfactory as a solution to the loss-of-traction, loss-of-braking and loss-of-control forces problem because: (a) they must be manually attached to the wheels, either prior to the vehicle leaving the departure point or at the point of entering snow covered road stretches; (b) they must be manually removed, once the conditions calling for their use in the first place, have changed to the degree that their use is no longer required, or is fact prohibited so as to preclude damage to the road surface; (c) they are relatively ineffective once the road surface is covered with frozen slush or smooth-worn but hard-frozen ice. In point of fact, the inconvenience of attaching and removing snow chains under inclement outdoors conditions is compounded by the dangers of lifting the vehicle wheels by means of hand-operated jack on the snow or slush covered roadside. This inconvenience and these dangers prompt potential users of snow-chains to avoid their use unless absolutely necessary or mandated by the police.